1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of manufacturing the liquid crystal display device.
2. Description of the Prior Art
Of the various types of liquid crystal display devices commercially available in the market, a liquid crystal display device utilizing an active matrix driving scheme, known as an active matrix drive liquid crystal display device, is largely developed. The active matrix drive liquid crystal display device employs thin-film transistors as switching elements and is well recognized superior in that, as compared with a liquid crystal display device utilizing a simple matrix driving scheme, a high contrast ratio can be attained regardless of the number of scanning electrodes and, hence, a clear, high-contrast image can be displayed at a high resolution.
While the active matrix drive liquid crystal display device is available in various types depending on the type of liquid crystal material employed, the active matrix drive liquid crystal display device employing twisted nematic liquid crystal has a normally white display mode. As is well known to those skilled in the art, the TN (twisted nematic) liquid crystal display device comprises a liquid crystal panel of a structure wherein liquid crystal material having 90.degree. twisted liquid crystal molecules is sandwiched between substrates, and a pair of polarizing plates with the liquid crystal panel sandwiched therebetween.
The TN liquid crystal display device having a normally white display mode is so designed that, while the direction of polarization of one polarizing plates is perpendicular to that of the other polarizing plate, one of the polarizing plates has its polarizing axis extending parallel or perpendicular to the major axis of the liquid crystal molecules held in contact with one of the substrates. In this normally white display mode of the TN liquid crystal display device, an image is displayed against a white background when no voltage is applied or a low voltage around a threshold value is applied while an image is displayed against a black background when a voltage higher than that is applied.
A mechanism of image display accomplished in the TN liquid crystal display device is such that, when a voltage is applied to the liquid crystal panel to cause the liquid crystal molecules, in a twisted state, to untwist, the liquid crystal molecules come to be oriented in a direction conforming to the direction of an electric field, permitting light passing across the liquid crystal panel to exhibit a different polarized characteristic with its transmittance modulated consequently. However, for a given orientation of the liquid crystal molecule, the polarization of light passing across the liquid crystal panel varies with a change in direction in which the light enters the liquid crystal display panel and, hence, the transmittance of light varies with a change in direction of incidence of the light upon the liquid crystal display panel. This means that the liquid crystal panel has an operating characteristic which is affected by characteristics of the viewing angle.
Specifically, in the normally white mode, the above discussed problem is considerable about when the black picture is displayed in which the liquid crystal molecules are set up relative to the substrates as a result of an application of the voltage. The characteristics of the viewing angle at this time are such as to be symmetric with respect to a plane containing the direction of the major axis of the liquid crystal molecules adjacent a center of the liquid crystal layer and perpendicular to the substrates, exhibiting a considerable change in transmittance of the light travelling towards such plane with a change in angle of incidence of the light upon the substrate. Accordingly, a change in characteristic of the viewing angle is particularly considerable in such direction.
In practice, however, since a rubbing is effected to the front substrate 1A, as viewed towards a display screen, in a direction shown by the arrow 18 in FIG. 8 and also to the rear substrate 1B in a direction shown by the arrow 19 in FIG. 8, the major axis of the liquid crystal molecules adjacent the center of the liquid crystal layer align on a plane perpendicular to any one of the front and rear substrates 1A and 2B, and therefore, the direction in which the change in characteristic of the viewing angle is considerable lies in a direction 34 heightwise of the display screen and symmetric in a horizontal direction as viewed towards the display screen.
The prior art TN liquid crystal display device of the type referred to above operating under the normally white mode has the following problems. Namely, if under the normally white mode the liquid crystal molecules completely set up in a direction perpendicular to the substrate surface upon application of the voltage, the display screen exhibit a true black color when viewed in a direction perpendicular to the substrate. This is because, when the major axis of the liquid crystal molecules lines up with each other in a direction parallel to the direction of travel of light, no optical phase difference occurs and the light passes across the liquid crystal layer with no polarized component thereof varied.
However, in practice, some of the liquid crystal molecules adjacent the substrate interface are hard to set up, even when the voltage is applied to a certain extent, due to an interaction thereof with the substrate. Also, even another portion of the liquid crystal molecules adjacent the center of the liquid crystal layer fails to set up and, therefore, no optical phase difference is eliminated with respect to the light travelling in a direction perpendicular to the substrate, failing to represent a true black color.
On the other hand, under such a molecule orientation, the light travelling in a direction substantially parallel to the direction of the major axis of some of the liquid crystal molecules adjacent the center of the liquid crystal exhibits a minimized optical phase difference as compared with the light travelling in a direction perpendicular to the substrate. Accordingly, in the actually manufactured liquid crystal panel, the characteristic of the viewing angle tends to become considerably asymmetric as shown in FIG. 9 on respective sides of the line normal to the substrate in a direction vertically of the display screen.
When the liquid crystal panel having the asymmetric characteristic as discussed above is employed in a video projection system, the following problem occurs.
In the case of the video projection system, it is generally recommended that while a projection lens employed having as small a F-number as possible, the projection lens receives the incoming light at a relatively large coverage, so that the image projected onto a screen becomes bright. This means that the liquid crystal panel as well should receive the incoming light at a large coverage. In order for the liquid crystal panel to receive light from the light source at a large coverage, the use must be made of a condensing lens for collecting the incoming light.
While a metal halide lamp is generally used as the light source in the video projection system, such a light source is not a point source of light and, therefore, it is difficult to provide a bundle of parallel rays of light. In view of the limitation imposed by the optics employed in the video projection system, the angle of incidence of light upon the liquid crystal panel and the coverage of light afforded by the liquid crystal panel vary from one local area of the liquid crystal panel to another.
Accordingly, where the characteristic of the viewing angle of the liquid crystal panel is asymmetric as discussed hereinbefore, the characteristic of the viewing angle of the liquid crystal panel varies from one local area to another, accompanied by a change in transmittance exhibited by the liquid crystal panel to such an extent that the image eventually displayed may have a varying brightness.
To improve the characteristic of the viewing angle, the following method is suggested.
Referring to FIG. 10, the alignment of the liquid crystal molecules 9 relative to the substrate 37 when the liquid crystal molecules 9 are rubbed in a direction shown by the arrow 35 is such that the liquid crystal molecules 9 contact the substrate 37 at a trailing side with respect to the direction of rubbing and are inclined upwardly and away from the substrate 37 at a angle .theta.. This angle .theta. of inclination of the liquid crystal molecules 9 relative to the substrate 37 is generally known as a pre-tilt angle 36.
FIG. 4 illustrates the direction of alignment processing effected to the liquid crystal panel by means of a rubbing technique. As shown therein, since the rubbing is effected to the front substrate 1A in the direction shown by the arrow 18 and to the rear substrate 1B in the direction shown by 19, if the alignment process is effected to the substrates 1A and 1B in such directions, the nematic liquid crystal molecules having not been twisted are, because of the pre-tilt of the liquid crystal molecules at the interface with each of the substrates, oriented so as to depict a helix 20 extending counterclockwise from the front substrate 1A towards the rear substrate 1B. On the other hand, if a chiral material having a clockwise twisting force is injected into the nematic liquid crystal and if the twisting force is strong, the result would be that the liquid crystal molecules at the interface with each of the substrates are oriented so as to depict a helix extending clockwise from the front substrate 1A towards the rear substrate 1B. It is, however, to be noted that the molecules at the interface with the substrate are oriented exhibiting a pre-tilt angle 30 conforming to the direction of orientation since they are subjected to an anchoring force resulting from the rubbing.
Under such an orientation, if portions of the liquid crystal molecules 9 touching the substrates 1A and 1B, respectively, have an equal pre-tilt angle 15 as shown in FIG. 11, the liquid crystal molecules 9 at the center 10 of the liquid crystal layer are not inclined and have their major axes extending parallel to any one of the substrates. In such case, application of an electric voltage between the substrates 1A and 1B results in an orientation of the liquid crystal molecules at the center 10 of the liquid crystal layer which incline as shown by +.theta.m39 and -.theta.m39, respectively, in FIG. 12 with an equal amount of energies given at the time of elastic deformation. Accordingly, this liquid crystal panel in the case under discussion will exhibit such a characteristic of viewing angle that, if the liquid crystal molecules set up at an angle of inclination of +.theta.m38, the black picture tend to sink at a position somewhat upwardly of the direction perpendicular to the substrate (this characteristic being hereinafter referred to as an upward viewing characteristic), but if they set up at an angle of inclination of -.theta.m39, the black picture tends to sink at a position somewhat downwardly of the direction perpendicular to the substrate (this characteristic being hereinafter referred to as a downward viewing characteristic).
The orientation of such upward and downward viewing characteristics exist alternating in minute regions and if these minute regions are uniformly distributed over the entire surface area of the liquid crystal panel, the characteristic of the viewing angle of this liquid crystal panel presents a mixture of the upward and downward viewing characteristics and gives an apparent symmetric characteristic in upward and downward directions. However, under such an orientation, it is random as to which one of molecule movements of one of the two equal energy conditions is assured at the time of application of the voltage and, therefore, the controllability is very instable. Therefore, the prior art liquid crystal panel is not suited for use in the liquid crystal display device.